As is well known, bags for the containing of fertilizers and livestock feeding materials are usually made from knitted PE or PP plastic yarn. In the knitting of plastic yarn, however, automatic tubular knitting machines are preferred over other types of knitting machines because of their high knitting rate. FIG. 5 is a conventional tubular knitting machine, which comprises a picking bowl carrying the pin and the wrap on the picking passage to form knitted fabric at the rotating element (a), and then, the cloth passes through the center of the upper and lower shaft. The rotating of the shafts (b and c) was achieved by the transmission mechanism (d) and caused the shaft (e) to rotate. At the top of said rotating shaft (e), a bevel gear (e1) is provided which engages with a corresponding bevel gear (f). Bevel gear (f), in turn, rotates the upper and lower shafts (b and c) after which the knitted cloth is rolled up on the rolling element (g). However, the speed of the press rollers (b and c) affects the density of knitting of the knitted belt. If the press rollers rotate too slowly, then the formed knitted belt has a high knitting density. Generally, in the interest of practicality and low manufacturing costs, the speed of the rotating shaft (b and c) must be kept constant, which means that the plastic yarn is first knitted and made into a bag by sewing across the knitted product to form a bag in a separate machine. However, the mouth and the base of the bags must be repeatedly knitted in order to better reinforce them, which means that additional procedures are required before such bags are completed. Hence, the traditional means of knitting these bags are very inconvenient and the manufacture thereof is uneconomical.